Soil Pipe Enlargement?

I have a 55-yr old slab house and would like to enlarge the soil pipe from 3" to 4" (the sewer line is 4") to be able to clean out the sewer pipe every couple of years. Before contacting a plumber for an estimate is this something that is even doable?

Anything is "doable" it just depends on how much money you want to spend. But even if you increased the size of the toilet pipe, it would probably still be difficult to snake the pipe from there. Spend your money on digging down to the main sewer and installing the proper cleanouts outside the building.